Oh — you must have the Zip. I forgot about that model.

The app warns you if the battery is getting low, and usually with a couple of days to spare. It’s easy to pop it in the charger while you shower or nap or something.

The FitBit you have came with a fabric wristband that you could use for sleep-tracking and alarms.

The One comes with a wrist band you can wear at night for the purpose of sleep tracking, and the alarm works just fine.